mod_admin_modules: Restrict module menu actions for core modules

This limits the newly added module actions(reinstall, uninstall, and update)
to only modules installed from the Zotonic Modules Repository.
core: Refactor exec_zmm/1 and the methods that use it

The exec_zmm/1 method changes to exec_zmm/3 and has a modularized interface
that can be easily extended to process subcommands.
Modules installed via the web module manager are installed in the global/default
module install path('priv/modules'). This way, the installed modules will be available to
 all the sites.